Speedometer Flaw

A buddy of mine just bought his first BMW, a really nice new 328......... But....... the speedometer is off 7 MPH, slow. He discovered this after getting passed consistently, then turned on his GPS and the speed was different. The dealer says that it is an acceptable variance of 10% and he will have to live with it. Well, it' ticks me off just hearing about it, now BMW has put a bad taste in his mouth and I don't say that I blame him. Any imput would be appreciated in order that I may relay any helpful info to him, however, I will be out place for a week or so, will check back in then.

The dealer says that it is an acceptable variance of 10% and he will have to live with it. Well, it' ticks me off just hearing about it, now BMW has put a bad taste in his mouth and I don't say that I blame him.

The dealer speaks truth. Manufacturers are obligated to provide instrumentation that will not mislead you into breaking the law. Many BMWs are blessed with speedometers that read closer to true speed than your friend's, but when they are off, they are off low. Getting worked up over it changes nothing.

Like most measuring instruments, it can be recalibrated, but any shop will be loath to do it (if they can/will at all) for liability reasons--if they make an error in calibration and you get a ticket, that's trouble. In any event, if it reads within BMW's required tolerance (+0-10%), they won't do it for free.

Your dealer is correct - a 10 percent variance is perfectly acceptable. There are all kinds of internet discussions on this issue, and all kinds of theories about it, but the biggest (and most rational reason for it that I hear for it) is this:

the speedometer variance keeps the speedometer generally accurate in case you change tire and wheel sizes, which occurs more often than not among private owners for any number of reasons. My car is 22 years old and it doesn't wear its stock "pie plate" wheels either. I'm running 16 inch E38 wheels instead, with spacers. Most tires, even ones of the same size, do vary slightly in their overall wheel and tire circumference. The difference is usually pretty minor if you keep to the widely accepted Plus One or Plus Two system (basically, this requires you to subtract 10 point from the tires profile size for every one inch increase in wheel size).

I should also add that your overall tire circumference doesn't remain static when driving. The friction created by the tires rotating against the road surface heats up the rubber, as well as the steel ply belts and the air inside the tire. As these items heat up, the tires will expand SLIGHTLY, particularly from the heating of the air inside the tires. This will result in a very minor increase in the tire's overall circumference. How much it will heat up and expand the tire depends on a large number of factors, including how fast you're going, the type of rubber compound your tires are made of, as well as the road surface and outside air temperature.

There's so many factors going on that it's impossible for BMW to guarantee a 100 percent accurate speedometer reading at all times. Even if they did calibrate it perfectly, it would immediately be off the first time you change to a different wheel and tire size.

I would also like to suggest not putting such a huge amount of confidence in a speed reading from a consumer GPS device. Consumer GPS devices are accurate to within an estimated 65 feet. 65 feet per second translates to 44.318 mph (calculated by multiplying 65 feet times 3600 [the number of seconds in an hour] by 5280 [the number of feet in a mile]. My point is the speed reading on a GPS is meant to be an estimate also, and is primarily meant to estimate the amount of time it takes you to travel a given distance in an estimated period of time. It's not a cop out -- it's the way the United States Government designed the GPS system, and for 99.99 percent of us, it's more than enough accuracy to get one's location, which is the whole point of the system, not to get ballpark speed measurements when your vehicle's speedometer is a whole lot more accurate.

Bad enough consumer GPS units aren't programmed to give you a better estimate than 65 feet of your position. What's worse is that if you're moving faster than 44 mph, that speed calcuation is already obsolete by the time it gives you a reading! For all your GPS unit knows, you might be 65 feet closer or further away than your actual position, or anywhere in between.

Personally, I think all things considered, 7 mph of variance isn't such a bad deal, and it's certainly a better offer than your GPS unit will give you.

Please check out my new science fiction adventure story now online at http://www.xybears.com. Your help and support with this would be greatly appreciated! Thank you!

I suppose it's a TUV law. Do Porsches, Audis, Mercedes, VW all show a -7% speed?
Explain why the speedo will vary depending on tires since the speed sensor is off the diff (on my car anyway). Just curious.

And you would think with sensors on every wheel the computer would know what size wheel you have. It already can tell if one wheel is rotating at a different speed than the other.

European Union member states must also grant type approval to vehicles meeting similar EU standards. The ones covering speedometers [4] [5][6] are similar to the UNECE regulation in that they specify that:

* The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
* The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

The standards specify both the limits on accuracy and many of the details of how it should be measured during the approvals process, for example that the test measurements should be made (for most vehicles) at 40, 80 and 120 km/h, and at a particular ambient temperature. There are slight differences between the different standards, for example in the minimum accuracy of the equipment measuring the true speed of the vehicle.

My 1990 Mazda MX6 was spot on MPH as compared to several rally computers.

Explain why the speedo will vary depending on tires since the speed sensor is off the diff (on my car anyway).

The indicated speed will not change but the actual speed will. The relationship between indicated and actual speeds naturally changes as well, which I think was the point.

Suppose you are driving at an indicated 60mph, on the correct wheel/tire size for the car, at the correct inflation pressure, and that this tire rolls at 833 revolutions per mile. The differential output shafts will also be turning at 833 rpm (one mi/min * 833 rev/mi). (The input shaft will be turning at 833 rpm times the final-drive ratio. Regardless of whether the speedo pickup is on an input or output shaft, that ratio is fixed and accounted for in the design, so we can ignore it for simplicity.)

Now suppose you fit tires spec'ed at 824 rev/mi and once again drive an indicated 60 mph. All the gearing is unchanged so we know that an indicated 60 mph still equates to 833 rpm at the output shafts. However, due to the larger-diameter tire (fewer revolutions per mile), you are now traveling 833/824 ~= 1.011 miles in that same minute, or 60.7 mph.

Those figures are not fictitious, they are real values for two minor variants (different load ratings) of 225/45R17 Pirelli PZero Rosso tires. That's a 1% variance from two fitments of what most people would consider to be the exact same tire make and model.

Now suppose you plus-size the rears (still PZero Rossos) to 255/35R18--a reasonably "correct" move because these sizes are both available as OE on a 335. That brings us down to 797 rev/mi, or 833/797 ~= 1.045 over indication = 62.7 mph.

All of the above assume nominal inflation pressures. Over-inflate the tires somewhat and you drift even further off the mark. This is why I advise people not to get worked up over speedometer "errors" of 5-10% at freeway speeds. The tolerances involved are not super-tight to begin with and normal operating variances, like the examples just given, affect the indication as much as or more than any built-in calibration error.

And you would think with sensors on every wheel the computer would know what size wheel you have. It already can tell if one wheel is rotating at a different speed than the other.

No, because there is no sensor that measures the rolling radius of the wheel. Differences in speed between wheels are trivial to detect because sensors provide independent measurement of rotational speed at each wheel. Linear speed is computed from rotational speed and rolling radius. Computing the rolling radius--wheel size--from rotational speed requires an independent measurement of linear speed--which the computer does not have.

I would also like to suggest not putting such a huge amount of confidence in a speed reading from a consumer GPS device. Consumer GPS devices are accurate to within an estimated 65 feet. 65 feet per second translates to 44.318 mph (calculated by multiplying 65 feet times 3600 [the number of seconds in an hour] by 5280 [the number of feet in a mile]. ...
Personally, I think all things considered, 7 mph of variance isn't such a bad deal, and it's certainly a better offer than your GPS unit will give you.

Your analysis is incorrect: you assume that there is a direct relationship between speed and positional accuracy. In fact, speed displayed by GPS units is an average of the distance between calculated positions. For instance, Garmin says that the speed it displays is <= 0.5 meters/second (about 1 mph): http://www8.garmin.com/products/nuvi/spec.html
Note that Garmin mentions that, with WAAS enabled, the speed accuracy is 0.05m/s (probably because successive positional deltas are smaller which enables better averaging.) Additionally, with SA turned off (and it will almost certainly never be turned on again), the positional accuracy is smaller than 65 ft (they quote it as typically <= 5 meters).

The indicated speed will not change but the actual speed will. The relationship between indicated and actual speeds naturally changes as well, which I think was the point.

Suppose you are driving at an indicated 60mph, on the correct wheel/tire size for the car, at the correct inflation pressure, and that this tire rolls at 833 revolutions per mile. The differential output shafts will also be turning at 833 rpm (one mi/min * 833 rev/mi). (The input shaft will be turning at 833 rpm times the final-drive ratio. Regardless of whether the speedo pickup is on an input or output shaft, that ratio is fixed and accounted for in the design, so we can ignore it for simplicity.)

Now suppose you fit tires spec'ed at 824 rev/mi and once again drive an indicated 60 mph. All the gearing is unchanged so we know that an indicated 60 mph still equates to 833 rpm at the output shafts. However, due to the larger-diameter tire (fewer revolutions per mile), you are now traveling 833/824 ~= 1.011 miles in that same minute, or 60.7 mph.

Those figures are not fictitious, they are real values for two minor variants (different load ratings) of 225/45R17 Pirelli PZero Rosso tires. That's a 1% variance from two fitments of what most people would consider to be the exact same tire make and model.

Now suppose you plus-size the rears (still PZero Rossos) to 255/35R18--a reasonably "correct" move because these sizes are both available as OE on a 335. That brings us down to 797 rev/mi, or 833/797 ~= 1.045 over indication = 62.7 mph.

All of the above assume nominal inflation pressures. Over-inflate the tires somewhat and you drift even further off the mark. This is why I advise people not to get worked up over speedometer "errors" of 5-10% at freeway speeds. The tolerances involved are not super-tight to begin with and normal operating variances, like the examples just given, affect the indication as much as or more than any built-in calibration error.

No, because there is no sensor that measures the rolling radius of the wheel. Differences in speed between wheels are trivial to detect because sensors provide independent measurement of rotational speed at each wheel. Linear speed is computed from rotational speed and rolling radius. Computing the rolling radius--wheel size--from rotational speed requires an independent measurement of linear speed--which the computer does not have.

Great analysis...

Additionally, a wheeel/tire setup at rest (0 RPM) has a different radius/diameter measurement than one rotating at highway speeds. That also factors into the equation. And, as the rotation increases, so does the diameter/radius measurement.

That's why, when recapped tires were common, you only saw a tire lose its "recaped" tread at relatively higher speeds .vs. sitting still.

The tire's rotation has a tendency to "flatten out" at higher speeds and expand the outside edge farther outward on the plane perpendicular to the rotational axis of the wheel/tire, increasing the tire's outside diameter measurement..

Garmin is a manufacturer of GPS devices, from which the United States government freely licenses access to the GPS system. Garmin does not administer or control the GPS system, and therefore has no say or input over the accuracy of the system. I think all of us would love to have a GPS system accurate to just one meter (3 feet), but unfortunately, the facts are going to get in the way of that little fantasy.

Previously, civilian GPS units were only accurate to within 300 feet, due to a requirement by the United States Department of Defense (which originally conceived, designed and implemented the system) that the signal be degraded for civilian applications. This system known as SA (Selective Availability), would purposely degrade the signal in GPS receivers that lacked a military decoder. This was to prevent the system from being used by an enemy against the United States military. SA was later rendered unnecessary, as the US military found a way to turn off the GPS system on a regional basis without affecting other legitimate users of the system. SA had largely been turned off by 2000 and is no longer in use.

In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive[24] declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset.

^ National Archives and Records Administration. U.S. Global Positioning System Policy. March 29, 1996.

The GPS system is far from perfect, and is subject to interference, errors, and malfunctions.

In automotive GPS receivers, metallic features in windshields,[90] such as defrosters or car window tinting films,[91] can act as a Faraday cage, degrading reception inside the car.

Man-made electromagnetic interference (EMI) can also disrupt or jam GPS signals. In one well-documented case it was impossible to receive GPS signals in the entire harbor of Moss Landing, California due to unintentional jamming caused by malfunctioning TV antenna preamplifiers.[92][93] Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range or line of sight. In 2002 a detailed description of how to build a short-range GPS L1 C/A jammer was published in the online magazine Phrack.[94]

The U.S. government believes that such jammers were used occasionally during the 2001 war in Afghanistan, and the U.S. military claims to have destroyed six GPS jammers during the Iraq War, including one that was destroyed with a GPS-guided bomb.[95] A GPS jammer is relatively easy to detect and locate, making it an attractive target for anti-radiation missiles. The UK Ministry of Defence| tested a jamming system in the UK's West Country on 7 and 8 June 2007.[96]

Some countries allow GPS repeaters, to facilitate the reception of GPS signals indoors and in obscured locations; however, under European Union and U.K. laws, these are prohibited because the signals can interfere with other GPS receivers that receive data from both satellites and the repeater.

Various techniques can address interference. One is to not rely on GPS as a sole source. According to John Ruley, "IFR pilots should have a fallback plan in case of a GPS malfunction".[97] Receiver Autonomous Integrity Monitoring (RAIM) is included in some receivers, to warn if jamming or another problem is detected. The U.S. military has also deployed since 2004 their Selective Availability / Anti-Spoofing Module (SAASM) in the Defense Advanced GPS Receiver (DAGR).[98] DAGR detects jamming and maintains its lock on encrypted GPS signals during interference.

The U.S. Government controls the export of some civilian receivers. All GPS receivers capable of functioning above 18 kilometers (11 mi) altitude and 515 metres per second (1,001 kn) [3] are classified as munitions (weapons) for which U.S. State Department export licenses are required. These limits attempt to prevent use of a receiver in a ballistic missile. They would not prevent use in a cruise missile since their altitudes and speeds are similar to those of ordinary aircraft.

This rule applies even to otherwise purely civilian units that only receive the L1 frequency and the C/A code and cannot correct for SA, etc.

Disabling operation above these limits exempts the receiver from classification as a munition. Vendor interpretations differ. The rule targets operation given the combination of altitude and speed, while some receivers stop operating even when stationary. This has caused problems with some amateur radio balloon launches, which regularly reach 30 kilometers (19 mi).

Complete information can be found here, complete with 3rd party references and footnotes can be found here.

Every BMW I've ever owned or driven has had a speedo that was around 4mph off (always high so that you thought you were going faster than you actually were). Its been that way since the 60's. I read once that BMW did that deliberately (whether its to comply with European expectations/standards or to get BMW drivers to drive slower is open for debate), though BMWde would never admit it. Some insiders spilled the beans anonymously. If you do a google search you'll see discussion of this for years back on all vintages of BMW. My Z4 is, acc. to my Garmin, 4 mph fast, so I compensate when I drive/set the cruise control.

As I mentioned before GPS is not an ideal or precise system for measuring speed

My Z4 is, acc. to my Garmin, 4 mph fast, so I compensate when I drive/set the cruise control.

How do you know if you're going 4 mph fast if your speedometer can be off by as much as 10 percent, and the GPS being a perfectly usable but less than precise system upon which to base one's speed? There are any number of things that can cause distortion, delay, or even a total malfunction in the system. I certainly would not consider that speed reading on your GPS system as always correct or infalliable.

A police officer with a radar gun or a stopwatch could give you a more accurate reading, although those methods too are also subject to possible error. Conversely, a GPS speed indication on a consumer GPS device is probably not going to be accurate enough to fight a speeding ticket, any more than a radar detector can be used to challenge the type of radar gun used.

Please check out my new science fiction adventure story now online at http://www.xybears.com. Your help and support with this would be greatly appreciated! Thank you!

Great analysis...
Additionally, a wheeel/tire setup at rest (0 RPM) has a different radius/diameter measurement than one rotating at highway speeds. That also factors into the equation. And, as the rotation increases, so does the diameter/radius measurement.

Thanks for the input guys. Regarding Garmin vs Speedometer
I have a new Benz E class and picked up a new Garmin GPS last week. The speed is spot on, no variance between speedo and GPS. (car has digital speed readout you can pull up also) I have been using Garmin marine navigational products for years, just upgraded boat units in spring and I really like them, hence purchasing portable unit for cars. My friend did make this comment to dealer "it's funny how the speedo in the Chevy is correct" and yours ?????? He has filed a complaint with the Attorney General. It's going to be an interesting story to follow. At this point, it's officially for sale if anyone wants a new 328 with 1100 miles and an incorrect speedo.

My friend did make this comment to dealer "it's funny how the speedo in the Chevy is correct" and yours ?????? He has filed a complaint with the Attorney General. It's going to be an interesting story to follow. At this point, it's officially for sale if anyone wants a new 328 with 1100 miles and an incorrect speedo.

First we went after sodas and candy bars in public schools. Then we went after McDonald's happy meals. Next we went after handgun owners in Chicago and Washington DC (both those cities trying to keep guns out of law abiding citizens, even though both cities regularly top the annual FBI crime list for the number of handgun murders in their city limits).

How do you know if you're going 4 mph fast if your speedometer can be off by as much as 10 percent, and the GPS being a perfectly usable but less than precise system upon which to base one's speed? There are any number of things that can cause distortion, delay, or even a total malfunction in the system. I certainly would not consider that speed reading on your GPS system as always correct or infalliable. A police officer with a radar gun or a stopwatch could give you a more accurate reading, although those methods too are also subject to possible error. Conversely, a GPS speed indication on a consumer GPS device is probably not going to be accurate enough to fight a speeding ticket, any more than a radar detector can be used to challenge the type of radar gun used.

Over the years I've compared the speedo reading with various different outside devices (dyno, speed gun, speed trap, other cars, etc.) and its been a pretty consistent 4 mph. BTW, usually the internal computer does register the correct speed. You can do a quick comparison by setting your cruise for a specific speed, say 60mph, then zeroing out the MPH in your OBC display, and then driving a bit to see what it registers. Again, at hiway speeds, I've found them to be off by about 4 mph.

BTW, this forum site has a nifty feature to show you other threads which have discussed the same issue; just scroll to the bottom of the page and you'll see at least 3 other threads which have discussed this same issue going back to 2008. The concensus is that BMW deliberately sets the speedo high and at hiway speeds its about 4 mph high.